Ink jet technology has become a technology of choice for printing documents and other digitally produced images on receiver members (e.g., paper and other media). In the ink jet process, described in more detail in Ink Jet Technology and Product Development Strategies by Stephen F. Ponds, and published by Torrey Pines Research in 2000, ink is jetted from an ink jet head that includes one or more ink jet nozzles onto a receiver member.
Contrasting with ink jet technology are other printing technologies, such as electrophotography and lithography. Lithography relies on the use of highly viscous inks in which pigment particles are dispersed with relatively small amounts of a fluid such as oil. Typically, the concentration of solids may exceed 90% by weight. The relatively small amount of solvent present in a lithographic print can be readily absorbed by the receiver member or treated using other suitable methods such as drying by heat, cross-linking, or overcoating with varnish.
Another advantage of the high viscosity inks used in lithography, is that the viscosity of the ink limits the ability of the ink to spread. Specifically, ink images often consist of sharp lines of demarcation, such as occur with alphanumeric symbols, halftone dots, edges of printed areas, etc. With high viscosity inks, the tendency of the ink to spread is minimized. This allows images on printed pages to have sharp edges and high resolution. It also reduces the tendency of ink to soak into relatively porous receiver members such as those that do not have a coating such as a clay overcoat. Examples of such receiver members include laser bond papers. If low viscosity ink soaks into the paper, paper fibers can show through. This limits the density of the printed image. Yet another advantage obtained with high viscosity inks is the minimization of halftone dot spread. This allows good gray scales to be produced and, for color images, allows images having a wide color gamut to be printed.
Yet another advantage of high viscosity inks such as those used in lithography is that such inks allow images to be printed on glossy papers such as those having a clay coating or polymer overcoat. Low viscosity inks tend to spread or run on these papers, adversely affecting various image quality parameters such as edge sharpness, resolution, and halftone dot integrity, and color balance.
U.S. Pat. No. 5,854,960 discloses a liquid electrophotographic engine having an inking roller, a squeegee to concentrate the liquid ink, and a photoreceptive member. In such apparatus, liquid electrophotographic ink is applied to an inking roller. The ink is then concentrated using the squeegee, preferably a squeegee in the form of a foam roller. This roller absorbs the clear solvent, leaving the marking particles in a concentrated ink. An electrostatic latent image is then formed on the photoreceptor and the latent image developed into a visible image by bringing the latent image bearing photoreceptor into contact with the concentrated ink bearing inking roller. The marking particles are then electrostatically attracted to the latent image sites on the photoreceptor. It should be noted that, during the ink concentration phase of this process, there is no image information in the ink so that image degradation during the concentration phase cannot occur.
U.S. Pat. No. 6,363,234 discloses a mechanism to concentrate liquid electrophotographic developer including a source of a gas that flows onto a surface containing a liquid developer image and a chamber adjacent to the source and the surface that receives the mixture.
Co-pending U.S. patent application Ser. No. 11/445,712 filed Jun. 2, 2006, discloses a digital printing press capable of producing prints at a high speed and high volume that utilizes ink jet technology, rather than an electrophotographic process, for applying the ink. In this type of apparatus, there is no electrostatic latent image formed on a photoreceptive or primary imaging member. In fact, there is no photoreceptive element and there is no electrostatic charge to attract marking particles to specific sites on the primary imaging member. Rather, small ink droplets, often with volumes as little as a few picoliters, are jetted or otherwise deposited strictly where a portion of the image is to be constructed.
As discussed in co-pending U.S. patent application Ser. No. 11/445,081 filed Jun. 2, 2006, the aforementioned problems associated with the dilute inks used in ink jet printing apparatus can be eliminated by first imaging by jetting the ink onto a primary imaging member, then concentrating the ink, and then transferring the concentrated ink to the receiver sheet such as paper. Alternatively, the concentrated ink can be transferred to a transfer intermediate member and then transferred from the transfer intermediate member to the receiver member.